Is it more than 30 days or 40 days in the dog days today?

Generally speaking, in everyone's impression, there are always 30 days before and after the dog days. But in fact, "Sanfu" also has a 40-day "Deluxe Edition". How to calculate the duration of "dog days" every year?

"Three days" is calculated according to the lunar calendar. Every summer to the third Geng Day in the future (the day with "Geng" in the trunk and branch calendar is called "Geng Day") is called "entering the ambush", and the second Geng Day after beginning of autumn is called "leaving the ambush". The length of "first stay" and "last stay" is fixed, both of which are 10 days. The length of the Mid-Autumn Festival is determined by the interval between the beginning of the autumn solar term and the first Geng Day after it. Therefore, the midrange may be 10 days, or it may be 20 days, corresponding to the "normal version" of 30 days and the "deluxe version" of 40 days.